Hydrodynamic actuating devices consisting of one prime mover, one torque converter and one rear-mounted transmission have been used for a long time in a multiplicity of different vehicles; the rear-mounted transmission being mostly designed as reversing power shift transmission in the case of construction machines. In a transmission of this kind, one power shift clutch is respectively provided for the forward gear and for the reverse gear.
One example of a reversing transmission switchable under load which has one hydrodynamic torque converter, one input shaft and one output shaft, one reversing set and gear clutches has been disclosed in the Applicant's DE A 198 46 955. This is a reversing transmission, especially suited for industrial lift trucks but also utilizable as an input assembly of a multi-step reversing transmission so as to be adequate for other construction machines like wheel loaders.
Power shiftable reversing transmissions have a series of advantages. They have compact construction and are comfortable to switch. In addition, the hydraulically actuatable gear clutches can be switched by modulating the switching pressure so that a controlled transfer of load is ensured. In certain construction machines and in fork lift trucks, together with the reversing operation, other conditions are important as starting off, inching and braking, for example.
For construction machines having wide range of motion, such as mobile cranes, one converter bridging clutch is installed in the torque converter and one stator freewheel is coordinated with the torque converter. The converter bridging clutch is designed so that at low traction requirements when no torque conversion is needed, it is closed in order to improve the degree of efficiency of the drive train.
With the hydrodynamic actuating devices described above, the maximum traction cannot be changed, especially not limited, since the traction in the stall point, i.e., when the output is stationary, depends only on the rotational speed of the prime mover. Under full load, the torque converter absorbs the maximum power for the drive system and converts a considerable amount of that in the oil to heat. In order that the hydraulic pump for the working actuating device can produce high conveying capacity, the prime mover has to be kept at high rotational speeds.
To limit the traction of such a hydrodynamic actuating device at high rotational speed of the prime mover, there are two possibilities. One consists in rear-mounting one secondary clutch on the torque converter, such as a directional clutch or providing a primary clutch between prime mover and torque converter (impeller of the torque converter).
For reasons of energy, one primary clutch is to be preferred to a secondary clutch. It has to be taken into consideration here that it is a secondary clutch and not a primary clutch which has to transmit a torque higher by the converter ratio.
A slipping clutch for traction limitation also serves to reduce the power that the torque converter absorbs. In construction machines, it is often necessary to simultaneously actuate the drive system and one oil pump dependent on the prime mover for driving the working hydraulic system and, at the same time, guide the power precisely to where it is needed.
The Applicant has produced so-called ergo-power transmissions with ergo-inch function which have a secondary clutch with which the traction is to be controlled to a small extent by way of a slipping directional clutch.
Power shift clutches, however, are not able to produce high switching capacities for a long time such as those particularly needed for wheel loaders. Therefore, a traction limitation function is ruled out.
A hydrodynamic actuating device with a torque converter in which one primary clutch and one converter bridging clutch are integrated has been proposed by the Caterpillar firm. For the control thereof two control valves are required which are independent of each other. The control electronics needed for their control must accordingly have two analog exits. The clutches integrated in a torque converter are, in addition, affected with the disadvantage that the converter inner pressure fluctuates strongly, can only be measured with difficulties and acts only upon one side of the respective switch piston of the primary clutch or of the converter bridging clutch. Furthermore, the converter inner pressure depends, to a great extent, on the driving rotational speed, the temperature and the characteristic line of the usually provided converter safety valve.
The problem on which the invention is based is to provide a hydrodynamic actuating device for construction machines of wide range of motion, especially a wheel loader, in which the traction at high rotational speed of the prime mover can be easily and reliably limited.